Method for producing powder mixture for powder metallurgy, and method for producing sintered body

ABSTRACT

The present invention provides a method for producing a powder mixture for powder metallurgy, which allows production of a green compact having both high density and high lubricity (low demolding force). The method, according to the present invention, for producing a powder mixture for powder metallurgy including an iron-base powder, a mechanical characteristic-improving powder and a lubricant, the method includes the steps of: mixing the iron-base powder and/or the mechanical characteristic-improving powder with an amide-based lubricant solution; removing a solvent in the solution; and coating the surface of the iron-base powder and/or the mechanical characteristic-improving powder with an amide-based lubricant.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for producing a powder mixture for powder metallurgy and a method for producing a sintered body using a powder mixture for powder metallurgy obtained by the method.

2. Description of the Related Art

In general, a powder mixture for powder metallurgy is produced by mixing an iron-base powder, as main raw materials, such as pure iron powder or iron-base alloy powder, a mechanical characteristic-improving powder, as a component for improving mechanical characteristics (e.g., strength property, processing characteristic, etc.) of a sintered body as final product, such as alloying powder or graphite powder, and a powdery lubricant (refer to, for example, Japanese Patent Application Laid-Open No. 2006-124777).

When the powder mixture for powder metallurgy is filled into a mold and compression-molded to form a green compact, the lubricant is added for reducing the frictional resistance between the green compact and the mold wall surface so that the green compact can be extracted from the mold with a low demolding force. The lubricant is considered to have also a definite action on the lubricity between powder particles during the rearrangement and densification of the iron-base powder or the mechanical characteristic-improving powder, which accompany the compression molding.

A sintered body which is obtained by sintering the green compact obtained by compression-molding the powder mixture for powder metallurgy is, in general, frequently required to have high-level mechanical characteristics including strength. The mechanical characteristics of the sintered body are considered to be greatly influenced by the density (degree of compaction index) of the green compact before sintering, although they are also dependent on the composition of the iron-base powder or the mechanical characteristic-improving powder as the main component. Namely, during the compression molding, the iron-base powder or mechanical characteristic-improving powder is rearranged and plastic-deformed. And as the green compact is more densified, or as voids within the green compact are reduced sufficiently, it is considered that the strength of the sintered body obtained with using this green compact is improved

The improvement in density of the green compact can be attained by optimizing (densifying) the shape, particle size and particle size distribution of iron-base powder. However, it is difficult to produce such an iron-base powder at industrially low cost. Further, the green compact can be densified by increasing the molding pressure during the compression molding. However, this method has a limitation in respect of the capability of molding equipment or the productivity of green compact.

These methods are not employed, but the addition amount of the lubricant may be reduced to improve more easily the density of the green compact. The reason is that the powdery lubricant used in the field of powder metallurgy is generally composed of an organic compound, and therefore the added lubricant is finally removed from the green compact due to thermal decomposition at a dewaxing step prior to sintering, however at the compression-molded stage, most of the lubricant remains between particles of the iron-base powder or the mechanical characteristic-improving powder within the green compact. Therefore, the lubricant consequently occupies a certain volume within the green compact, constituting a factor of disturbing the densification of the green compact.

However, when the addition amount of the lubricant is reduced, the frictional resistance between the green compact and the mold wall surface increases, and the demolding force in the extraction of the green compact from the mold is consequently increases, causing mold damage such as mold galling. Further, the lubricity between powder particles during the rearrangement of the iron-base powder or the like which accompanies compression molding is also deteriorated, limiting the improvement in density of the green compact.

As described above, it is difficult to ensure both lubricity (low demolding force) and high density in the green compact.

Further, a sintered body having good appearance and characteristics is difficult to be obtained from the powder mixture for powder metallurgy containing powdery lubricant. The reason is that, at the production of the powder mixture for powder metallurgy, the powdery lubricant exists in a dispersed state within the powder mixture, however, in the course of handling to compression molding operation, the lubricant approaches or contacts with each other, and agglomerates due to an intermolecular force or other effects, and the particle size may become larger than that of the lubricant added in early stage. When the agglomerated lubricant emerges on the surface of the green compact, it becomes the blot on the surface of the sintered body. And when it remains within the green compact, it becomes the surface roughness or internal defect of the sintered body.

Further, this powder mixture for powder metallurgy involves segregation of the mechanical characteristic-improving powder such as alloying powder or graphite powder.

SUMMARY OF THE INVENTION

In view of the circumstances as described above, the present invention is accomplished and has an object to provide a method for producing a powder mixture for powder metallurgy, which can inhibit segregation of the mechanical characteristic-improving powder while giving high lubricity to powder particles in the powder mixture for powder metallurgy, and thus can allow production of a green compact having high density and yet high lubricity (low demolding force) and further production of a sintered body with lesser surface blot, surface roughness or internal defect.

Namely, as the method for solving the above object, the present invention provides a method for producing a powder mixture for powder metallurgy including an iron-base powder, a mechanical characteristic-improving powder and a lubricant, the method includes the steps of: mixing the iron-base powder and/or the mechanical characteristic-improving powder with an amide-based lubricant solution; removing a solvent in the solution; and coating the surface of the iron-base powder and/or the mechanical characteristic-improving powder with an amide-based lubricant.

According to this structure, the lubricant can be easily spread over the whole surface of the iron-base powder or the mechanical characteristic-improving powder since the lubricant is added to these powders in a solution state.

The expression “coating the surface of powder with the amide-based lubricant” herein includes the form of covering a part of the surface of powder with the amide-based lubricant in addition to the form of covering the whole surface of powder with the amide-based lubricant.

In the method for producing a powder mixture for powder metallurgy of the present invention, it is preferable that 0.01 to 2.0 parts by mass of the amide-based lubricant is added to 100 parts by mass of total amount of the iron-base powder and the mechanical characteristic-improving powder.

The amide-based lubricant is preferably composed of an aliphatic amide and/or an aliphatic bisamide. Concretely, it is preferable that the aliphatic amide is N-oleyl palmitic amide, and the aliphatic bisamide is ethylene-bis-oleic amide.

The mechanical characteristic-improving powder is preferably composed of at least one element selected from the group consisting of copper, nickel, chromium, molybdenum, graphite and manganese sulfide.

The prevent invention includes a method for producing a sintered body, including the step of: compression-molding the powder mixture for powder metallurgy obtained by the above producing methods; and sintering.

In this case, the compression molding is preferably performed at ordinary temperature.

According to the present invention, a powder mixture for powder metallurgy which allows production of a green compact with high density and low demolding force can be obtained.

PREFERRED EMBODIMENTS OF THE PRESENT INVENTION

The method for producing a powder mixture for powder metallurgy according to the present invention, is a method for producing the powder mixture including an iron-base powder, a mechanical characteristic-improving powder and a lubricant, and the method includes the steps of mixing the iron-base powder and/or the mechanical characteristic-improving powder with an amide-based lubricant solution, removing a solvent in the solution, and coating the surface of the iron-base powder and/or the mechanical characteristic-improving powder with an amide-based lubricant.

In the method for producing a powder mixture for powder metallurgy of the present invention, since a solution in which the lubricant is dissolved is used to dispose the lubricant onto the powder surface, the lubricant can be easily spread over the whole surface of the powder. Therefore, the coating of the powder surface with the lubricant can be easily preformed, and a uniform coating state can be easily ensured. The addition amount of the lubricant can be easily adjusted for uniformly disposing the lubricant over the whole powder surface. Thus, the present invention has the following advantages.

(Production of Densified Green Compact and Inhibition of Segregation of Mechanical Characteristic-Improving Powder)

Namely, in a powder mixture for powder metallurgy obtained by the above-mentioned method, since the surface of the iron-base powder or the mechanical characteristic-improving powder is coated with the lubricant, the lubricity between powder particles is improved. Therefore, the iron-base powder or the mechanical characteristic-improving powder is easily rearranged in compression molding.

Even if the mechanical characteristic-improving powder is smaller in specific gravity than the iron-base powder, for example, and easy to be liberated and segregated in the powder mixture, it is estimated that the lubricant existing on the particle surface serves as a binder to allow the mechanical characteristic-improving powder adhere to the iron-base powder surface, and therefore the mechanical characteristic-improving powder can be inhibited from being segregated within the powder mixture.

Further, the lubricant never hinders the densification of green compact since it exists, within the powder mixture for powder metallurgy, not as a powder but in the form of a film between the particles of the iron-base powder or the mechanical characteristic-improving powder. The green compact is rather densified since the lubricant film is easily deformed and moved by a stress in the compression molding, and pushed out of between the powder particles.

As described above, the powder mixture for powder metallurgy obtained by the method of the present invention allows production of a green compact with high density, since the iron-base powder or the mechanical characteristic-improving powder is easily rearranged in the compression molding, and the residual quantity of the lubricant in the green compact can be also kept low. Further, prevention of segregation of the mechanical characteristic-improving powder can be expected.

(Production of Green Compact Excellent in Lubricity)

According to the method of the present invention, since the lubricant is adhered to the surface of the iron-base powder or the mechanical characteristic-improving powder, the lubricant exists also on the surface (the interface with the mold) of a resulting green compact to reduce the frictional resistance with the mold.

Further, since the lubricant pushed out of between the powder particles by the stress in the compression molding is leaked to the surface of the green compact to thinly cover the surface, the frictional resistance with the mold is further more reduced.

Therefore, the powder mixture for powder metallurgy obtained by the method of the present invention allows production of a green compact excellent in lubricity and reduced in demolding force.

(Production of Sintered Body Excellent in Appearance)

According to the production method of the present invention, since the lubricant exists in a state in which it is adhered onto the surface of the iron-base powder or the mechanical characteristic-improving powder, the lubricant is inhibited from aggregating in the course of handling before compression molding and being exposing to the outer surface of the green compact or staying within the green compact.

Therefore, the powder mixture for powder metallurgy obtained by the production method of the present invention allows the production of a sintered body with lesser surface blot, surface roughness or internal defect.

The method for producing a powder mixture for powder metallurgy of the present invention will be then described in detail.

[Production Method of Powder Mixture for Powder Metallurgy]

(Iron-Base Powder)

Examples of the iron-base powder used in the present invention include pure iron powder and iron-base alloy powder.

The pure iron includes an iron powder containing 97% by mass or more of iron powder, with the balance being inevitable impurities (e.g., oxygen, silicon, carbon, manganese, etc.), which can be substantially regarded as a pure iron component.

The iron-base alloy powder contains, as a component other than iron, alloy components such as copper, nickel, chromium, molybdenum, sulfur, or manganese for improving characteristics of sintered body. The iron-base alloy powder is roughly classified to a partially alloyed powder (produced by diffusion-joining the alloy element to base iron powder) and a prealloyed powder (produced by adding the alloy element in dissolution step).

In the present invention, these iron-base powders may be used singly or in combination of two or more kinds thereof.

The iron-base powder can be produced, for example, by making molten iron (or molten iron alloy) into fine particles by atomization and then performing reduction and pulverization. An iron-base powder obtained by such a production method has a particle size (median diameter) of about 20 to 25 μm, which corresponds to 50% accumulated particle size distribution in particle size distribution evaluated by sieving method. In the present invention, an iron-base powder with a particle size (median diameter) of about 50 to 150 μm (micro track method) is preferably used.

(Mechanical Characteristic-Improving Powder)

As the mechanical characteristic-improving powder used in the present invention, any powder, for example, metal powder, inorganic powder or the like, can be used without limitation, as long as it can improve mechanical characteristics such as hardness and toughness of sintered body or can enhance machinability thereof by diffusing into the iron-base powder in sintering to be subsequently performed.

Examples of the metal powder include copper, nickel, chromium, molybdenum, tin, vanadium, manganese, and ferro phosphorus. Particularly in case where pure iron powder is used as the iron-base powder, it is preferred to add such a metal powder. The metal powder may be composed of a ferroalloy alloyed with iron, or an alloy powder composed of two kinds or more of elements other than iron.

Examples of the inorganic powder include sulfide such as manganese sulfide or manganese disulfide; nitride such as boron nitride; an oxide such as boric acid, magnesium oxide, potassium oxide, or silicon oxide; graphite such as natural graphite or artificial graphite; phosphorus, sulfur, and the like.

The above-mentioned mechanical characteristic-improving powders may be used singly or in combination of two or more kinds thereof.

In the present invention, a mechanical characteristic-improving powder having a particle size (median diameter) of about 2 to 150 μm (micro-track method) is preferably used.

The total mixing amount of the mechanical characteristic-improving powders is preferably set to 0.1 parts by mass or more but not exceeding 10 parts by mass relative to 100 parts by mass of the iron-base powder, although it can be optionally determined according to various characteristics required in a final product (sintered body) without particular limitation. In case of the above-mentioned range, the mechanical characteristic-improving component can sufficiently diffuse into the iron-base powder, and therefore the mechanical characteristic-improving effect can be developed. Further, deterioration of compaction property can be prevented to ensure a green compact having a sufficient density. Particularly for the mixing amount of the inorganic powder such as graphite, it is preferable to set to 0.1 parts by mass or more but not exceeding 10 parts by mass relative to 100 parts by mass of the iron-base powder. In case of the above-mentioned range, the addition effect can be sufficiently developed while preventing the deterioration of the mechanical characteristics of sintered body due to the adverse effect of the mixing of the inorganic powder.

When pure iron powder is used as the iron-base powder, for example, the preferable mixing amount of each mechanical characteristic-improving powder relative to 100 parts by mass of the pure iron powder is such that copper: 0.1 to 5 parts by mass, nickel: 0.1 to 10 parts by mass, chromium: 0.1 to 8 parts by mass, molybdenum: 0.1 to 5 parts by mass, graphite: 0.1 to 2 parts by mass, or manganese sulfide: 0.1 to 5 parts by mass.

(Lubricant)

In the present invention, the powder mixture for powder metallurgy is prepared by use of an amide-based lubricant as the lubricant, whereby a green compact with high density and excellent lubricity and a sintered body with excellent appearance can be obtained.

Examples of the amide-based lubricant used in the present invention include aliphatic amide and aliphatic bisamide.

Examples of the aliphatic amide include lauric amide, palmitic amide, stearic amide, behenic amide, hydroxy stearic amide, oleic amide, erucamide, ricinoleic amide, N-lauryl lauric amide, N-palmityl palmitic amide, N-stearyl stearic amide, N-oleyl oleic amide, N-stearyl oleic amide, N-oleyl stearic amide, N-stearyl erucamide, N-oleyl palmitic amide, N-stearyl-hydroxy stearic amide, N-oleyl-hydroxy stearic amide, methylol stearic amide, and methylol behenic amide.

Examples of the aliphatic bisamide include methylene-bis-stearic amide, methylene-bis-lauric amide, methylene-bis-hydroxy stearic amide, ethylene-bis-caprylic amide, ethylene-bis-capric amide, ethylene-bis-lauric amide, ethylene-bis-stearic amide, ethylene-bis-isostearic amide, ethylene-bis-hydroxy stearic amide, ethylene-bis-behenic amide, hexamethylene-bis-stearic amide, hexamethylene-bis-behenic amide, hexamethylene-bis-hydroxy stearic amide, butylene-bis-hydroxy stearic amide, N,N′-distearyl adipic amide, N,N′-distearyl sebacic amide, methylene-bis-oleic amide, ethylene-bis-oleic amide, ethylene-bis-erucamide, hexamethylene-bis-oleic amide, N,N′-dioleyl adipic amide, N,N′-dioleyl sebacic amide, m-xylylene stearic amide, and N,N′-distearyl isophthalic amide.

These amide-based lubricants may be used singly or in combination of two or more kinds thereof. It is particularly preferred to use N-oleyl palmitic amide (melting point: about 62° C.) as the aliphatic amide, and ethylene-bis-oleic amide (melting point: about 120° C.) as the aliphatic bisamide. In the present invention, since the amide-based lubricant is added to the iron-base powder and the like not in a melt state but in a solution state, such an amide-based lubricant having a high melting point can be used.

The amide-based lubricant is preferably contained in the powder mixture for powder metallurgy within the range of 0.01 to 2.0 parts by mass relative to 100 parts by mass of the total of the iron-base powder and the mechanical characteristic-improving powder. When the content of the amide-based lubricant is within the above-mentioned range, the effects by the addition of lubricant (application of lubricity to powder particles and densification of green compact) can be sufficiently exhibited without the risk of inhibiting the densification of green compact by the addition of lubricant. The content of the amide-based lubricant is set more preferably to 0.05 to 1.2 parts by mass, further more preferably to 0.1 to 1.0 part by mass.

(Lubricant Solution)

The present invention is characterized by using a solution in which the amide-based lubricant is dissolved to coat the surface of the iron-base powder and the mechanical characteristic-improving powder with the lubricant.

In the present invention, the solvent used to prepare the amide-based lubricant solution is not particularly limited as long as the amide-based lubricant can dissolve into the solvent, and the solvent can be easily removed by heating or decompression after mixed with the iron-base powder and the like. Examples of such a solvent includes aromatic hydrocarbon such as benzene or toluene; and alcohol such as methanol, ethanol, isopropanol, n-butanol, isobutanol, sec-butanol, t-butanol, pentanol, ethylene glycol, propylene glycol, or 1,4-butane diol. These solvents may be used singly or in combination of two or more kinds thereof.

A known method can be used to prepare the amide-based lubricant solution. For example, the amide-based lubricant solution can be prepared by adding the solvent to the amide-based lubricant followed by stirring. When the amide-based lubricant is hardly soluble to the solvent, the preparation can be performed while properly heating the solvent. When toluene, for example, is used as the solvent, the toluene is heated to about 60° C.

The addition amount of the amide-based lubricant to the solvent is not particularly limited, but it is preferable that the content of the amide-based lubricant in the powder mixture for powder metallurgy is within the above-mentioned range, the amide-based lubricant solution can be uniformly spread over the whole surface of the iron-base powder or the mechanical characteristic-improving powder, and then rapidly removing the solvent. For example, the addition amount relative to 100 parts by mass of the solvent is set to preferably 1 part by mass or more, more preferably 2 parts by mass or more and to preferably 40 parts by mass or less, more preferably 25 parts by mass or less.

(Coating with Amide-Based Lubricant)

The coating of the surface of the iron-base powder or the mechanical characteristic-improving powder with the amide-based lubricant by use of the amide-based lubricant solution can be performed by a known method.

For example, the coating can be performed by mixing the amide-based lubricant solution to the iron-base powder, removing the solvent (by heating or decompression), adding the mechanical characteristic-improving powder thereto, and mixing, or by mixing the amide-based lubricant solution to the mechanical characteristic-improving powder, removing the solvent, and adding the iron-base powder thereto, and mixing. According to this, only one of the iron-base powder and the mechanical characteristic-improving powder can be covered with the amide-based lubricant.

The coating can be performed also by mixing the iron-base powder to the amide-based lubricant solution, and further adding the mechanical characteristic-improving powder thereto, and mixing, or by mixing the amide-based lubricant solution to a mixture of the iron-base powder and the mechanical characteristic-improving powder, and then removing the solvent. According to this, both the iron-base powder and the mechanical characteristic-improving powder can be coated with the amide-based lubricant.

In the present invention, since it is preferred to improve the lubricity between powder particles to promote the rearrangement of the powder particles in the compression molding, the method for coating both the iron-base powder and the mechanical characteristic-improving powder with the amide-based lubricant is preferably adopted.

The addition amount of the amide-based lubricant solution to the iron-base powder or the mechanical characteristic-improving powder is set, relative to 100 parts by mass of the iron-base powder or the mechanical characteristic-improving powder, to preferably 1 part by mass or more, more preferably 5 parts by mass or more, and to preferably 20 parts by mass or less, more preferably 10 parts by mass or less, although it depends on the density of the amide-based lubricant in the solution. According to this, the amide-lubricant solution can be spread over the whole surface of the iron-base powder or the mechanical characteristic-improving powder while keeping the content of the amide-based lubricant in the powder mixture for powder metallurgy within the above-mentioned range, and the solvent can be rapidly removed thereafter.

The mixing of the iron-base powder or the mechanical characteristic-improving powder with the amide-based lubricant solution can be performed by use of a generally-used mixer such as a mixer with blade, a V-shape mixer, or a double conical mixer (W cone). A high-speed mixer of a type capable of mixing and stirring contents by a rotary blade and further performing heating or decompression is preferably used.

[Method for Producing Sintered Body]

The present invention includes also a method for producing a sintered body by compression-molding the powder mixture for powder metallurgy obtained by the above-mentioned method followed by sintering.

(Compression Molding)

In the present invention, a known method can be adopted to perform the compression molding of the powder mixture for powder metallurgy without particular limitation. Concrete molding conditions such as molding temperature and molding pressure are varied depending on the kinds or addition amounts of the components constituting the powder mixture or the shape of green compact. The powder mixture for powder metallurgy obtained by the method of the present invention is used for the compression molding, and a green compact having a density of about 6.85 g/cm³ or more can be consequently produced. The molding temperature in the compression molding in the present invention is preferably room temperature.

In the present invention, the powder mixture for powder metallurgy obtained by the above-mentioned method is used for compression molding, and the demolding pressure of the green compact can be consequently reduced. When the compression is performed at a molding pressure of 490.3 MPa (5 t/cm²), the concrete demolding pressure can be made to 10.0 MPa or less, more preferably to 9.5 MPa or less, further preferably to 9.0 MPa or less. When the compression is performed at a molding pressure of 686.5 MPa (7 t/cm²), the demolding pressure can be made to 15.0 MPa or less, more preferably to 14.0 MPa or less.

(Dewaxing)

In the present invention, it is preferred to include, prior to sintering of the green compact obtained by the compression molding, a dewaxing step of removing the amide-based lubricant remaining within the green compact. Such a dewaxing step can be performed, for example, by heating the green compact to thermally decompose the amide-based lubricant within the green compact. The heating condition in the dewaxing step can be appropriately adjusted according to the kind of the amide-based lubricant used, and in general, simple heating at about 150 to 200° C. for 10 to 30 minutes (more suitably 15 to 20 minutes) is sufficient.

(Sintering)

The method obtaining a sintered body with using the above-mentioned green compact is not particularly limited, and the general sintering method can be adopted. Concrete sintering conditions are varied depending on the kinds or addition amounts of the components constituting the green compact, the kind of final products, or the like. The sintering is preferably performed, for example, in an atmosphere of N₂, N₂−H₂, hydrocarbon or the like, at a temperature of about 1000 to 1300° C. for 5 to 60 minutes.

EXAMPLES

The present invention will be further described in detail based on Examples. The present invention is never limited to the following examples, and all variations and modifications made within the range not departing from the gist of the present invention are included in the technical scope of the present invention. “Part” and “%” referred to in Examples mean “part by mass” and “% by mass” respectively unless a notice is given thereto.

Evaluation methods used in Examples will be described first.

(Green Compact Density)

The green compact density was calculated based on mass/volume by measuring the mass of a powder mixture for powder metallurgy, and measuring, after compression molding thereof, the diameter and length of a green compact (cylinder) by a micrometer to determine the volume.

(Demolding Pressure)

An Instron tester was used to measure the demolding pressure in extraction of a green compact from a mold after molding.

Example 1 Preparation of Amide-Based Lubricant Solution

An amide-based lubricant solution was prepared by adding, to 20 parts of toluene as the solvent, 4 parts of N-oleyl palmitic amide (by NIPPON FINE CHEMICAL) as the amide-based lubricant, and mixing them with heated to about 60° C.

<Preparation of Powder Mixture for Powder Metallurgy>

The obtained amide-based lubricant solution was added and mixed to 100 parts of pure powder (by KOBE STEEL, 300 M, particle size 50-150 μm) as the iron-base powder, and then 2.0 parts of copper powder (MAKIN METAL POWDERS, AHF-100, particle size 30-50 μm), and 0.8 parts of graphite powder (by NIPPON GRAPHITE INDUSTRIES, JCPBK, particle size 3-5 μm), as the mechanical characteristic-improving powders, were added and mixed thereto, and the toluene was distilled under reduced pressure, and a powder mixture for powder metallurgy was consequently obtained (the addition amount of the amide-based lubricant based on 100 parts in total of the iron-base powder and the mechanical characteristic-improving powder: 0.3 part).

<Compression Molding>

To a metal mold (cylindrical mold, 11.28φ), 7.0 g of the powder mixture for powder metallurgy was put, and compression molding was performed thereto, at room temperature (25° C.), at surface pressures of 490.3 MPa (5 t/cm²) or 686.5 MPa (7 t/cm²), and thereby green compact 1-1 and green compact 1-2 were obtained respectively.

<Characteristics of Green Compact>

The density and demolding pressure in the extraction from the mold were measured for each of the resulting green compacts 1-1 and 1-2. The results are shown in Table 1.

Examples 2 and 3 Preparation of Amide-Based Lubricant Solution, Preparation of Powder Mixture for Powder Metallurgy, and Compression Molding

Green compacts 2-1 and 2-2 (Example 2) and green compacts 3-1 and 3-2 (Example 3) were produced in the same manner as in Example 1, except that the amide-based lubricant solution was added so that the addition amount of the amide-based lubricant is 0.4 part (Example 2) or 0.5 part (Example 3) based on 100 parts in total of the iron-base powder and mechanical characteristic-improving powder.

<Characteristics of Green Compact>

The density and demolding pressure in the extraction from the mold were measured for each of the obtained green compacts 2-1 and 2-2 and green compacts 3-1 and 3-2. The results are shown in Table 1.

Example 4 Preparation of Amide-Based Lubricant Solution, Preparation of Powder Mixture for Powder Metallurgy, and Compression Molding

Green compacts 4-1 and 4-2 were produced in the same manner as in Example 1, except that ethylene-bis-oleic amide (by DIC) was used as the amide-based lubricant in stead of N-oleyl palmitic amide used in Example 1.

<Characteristics of Green Compact>

The density and demolding pressure in the extraction from the mold were measured for each of the obtained green compacts 4-1 and 4-2. The results are shown in Table 1.

Examples 5 and 6 Preparation of Amide-Based Lubricant Solution, Preparation of Powder Mixture for Powder Metallurgy, and Compression Molding

Green compacts 5-1 and 5-2 (Example 5) and green compacts 6-1 and 6-2 (Example 6) were produced in the same manner as in Example 4, except that the amide-based lubricant solution was added so that the addition amount of the amide-based lubricant was 0.4 part (Example 5) or 0.5 part (Example 6) based on 100 parts in total of the iron-base powder and the mechanical characteristic-improving powder.

<Characteristics of Green Compact>

The density and demolding pressure in the extraction from the mold were measured for each of the obtained green compacts 5-1 and 5-2 and green compacts 6-1 and 6-2. The results are shown in Table 1.

Example 7 Preparation of Powder Mixture for Powder Metallurgy

As the amide-based lubricant, N-oleyl palmitic amid powder was added and mixed to 100 parts of pure iron powder as the iron-base powder, and 2.0 parts of copper powder and 0.8 parts of graphite powder, as the mechanical characteristic-improving powder, were added and mixed thereto, and thereby a powder mixture for powder metallurgy was obtained (the addition amount of the amide-based lubricant based on 100 parts in total of the iron-base powder and the mechanical characteristic-improving powder: 0.3 part).

Namely, the powder mixture for powder metallurgy was produced in the same manner as in Example 1 except that the amide-based lubricant powder (powder of N-oleyl palmitic amide) was used instead of the amide-based lubricant solution of Example 1.

<Compression Molding>

Green compacts 7-1 and 7-2 were produced by performing the same compression molding as in Example 1 with use of the obtained powder mixture for powder metallurgy.

Characteristics of Green Compact>

The density and demolding pressure in the extraction from the mold were measured for each of the obtained green compacts 7-1 and 7-2. The results are shown in Table 2.

Examples 8 and 9 Preparation of Powder Mixture for Powder Metallurgy and Compression Molding

Green compacts 8-1 and 8-2 (Example 8) and Green compacts 9-1 and 9-2 (Example 9) were produced in same manner as in Example 7, except that the amide-based lubricant powder was added so that the addition amount of the amide-based lubricant was 0.4 part (Example 8) or 0.5 part (Example 9) based on 100 parts in total of the iron-base powder and the mechanical characteristic-improving powder.

<Characteristics of Green Compact>

The density and demolding pressure in the extraction from the mold were measured for each of the obtained green compacts 8-1 and 8-2 and green compacts 9-1 and 9-2. The results are shown in Table 2.

Example 10 Preparation of Powder Mixture for Powder Metallurgy and Compression Molding

Green compacts 10-1 and 10-2 (Example 10) were produced in the same manner as in Example 7, except that powder of ethylene-bis-oleic amide was used as the amide-based lubricant instead of the powder of N-oleyl palmitic amide used in Example 7.

Namely, the green compacts were produced in the same manner as Example 4, except that the amide-based lubricant powder (the powder of ethylene-bis-oleic amide) was used instead of the amide-based lubricant solution of Example 4.

<Characteristics of Green Compact>

The density and demolding pressure in the extraction from the mold were measured for each of the obtained green compacts 10-1 and 10-2. The results are shown in Table 2.

Examples 11 and 12 Preparation of Powder Mixture for Powder Metallurgy and Compression Molding

Green compacts 11-1 and 11-2 (Example 11) and green compacts 12-1 and 12-2 (Example 12) were produced in the same manner as in Example 10, except that the amide-based lubricant solution was added so that the addition amount of the amide-based lubricant was 0.4 part (Example 11) or 0.5 part (Example 12) based on 100 parts in total of the iron-base powder and the mechanical characteristic-improving powder.

<Characteristics of Green Compact>

The density and demolding pressure in the extraction from the mold were measured for each of the obtained green compacts 11-1 and 11-2 and green compacts 12-1 and 12-2. The results are shown in Table 2.

TABLE 1 Characteristics of Green Compact Surface Pressure 5 t/cm² Surface Pressure 7 t/cm² Content of Green Compact Demolding Green Compact Demolding Amide-based Lubricant Lubricant Density Pressure Density Pressure Example (Toluene Solution) (Part) (g/cm³) (Mpa) (g/cm³) (Mpa) 1 N-oleyl Palmitic 0.3 6.90 9.63 7.22 14.27 2 Amide (PNT) 0.4 6.92 9.50 7.21 13.82 3 0.5 6.90 8.52 7.19 11.37 4 Ethylene-bis-oleic 0.3 6.94 8.96 7.26 13.31 5 Amide (OBA-N) 0.4 6.94 9.35 7.25 12.44 6 0.5 6.93 8.20 7.20 10.59

TABLE 2 Characteristics of Green Compact Surface Pressure 5 t/cm² Surface Pressure 7 t/cm² Content of Green Compact Demolding Green Compact Demolding Amide-based Lubricant Lubricant Density Pressure Density Pressure Example (Powder) (Part) (g/cm³) (Mpa) (g/cm³) (Mpa) 7 N-oleyl Palmitic 0.3 6.86 12.03 7.19 17.41 8 Amide (PNT) 0.4 6.86 11.52 7.18 15.40 9 0.5 6.87 9.48 7.11 10.10 10 Ethylene-bis-oleic 0.3 6.91 11.79 7.22 17.27 11 Amide (OBA-N) 0.4 6.91 11.33 7.21 15.87 12 0.5 6.94 9.59 7.21 11.89

It was found from the comparisons of Examples 1 to 6 with Examples 7 to 12 that by preparing the powder mixture for powder metallurgy with use of the solution of amide-based lubricant, the green compact density (strength) is enhanced, and the demolding pressure is reduced, compared with the case of using the powder of amide-based lubricant. Further, the results of Examples 1 to 6 show that the larger the addition amount of the amide-based lubricant is, the lower the demolding pressure is.

INDUSTRIAL AVAILABILITY

The present invention is useful for production of a sintered body with high strength and good appearance. 

1. A method for producing a powder mixture for powder metallurgy including an iron-base powder, a mechanical characteristic-improving powder and a lubricant, the method comprising the steps of: mixing the iron-base powder and/or the mechanical characteristic-improving powder with an amide-based lubricant solution; removing a solvent in the solution; and coating the surface of the iron-base powder and/or the mechanical characteristic-improving powder with an amide-based lubricant.
 2. The method for producing the powder mixture for powder metallurgy according to claim 1, wherein 0.01 to 2.0 parts by mass of the amide-based lubricant is added to 100 parts by mass of total amount of the iron-base powder and the mechanical characteristic-improving powder.
 3. The method for producing a powder mixture for powder metallurgy according to claim 1, wherein the amide-based lubricant is composed of an aliphatic amide and/or an aliphatic bisamide.
 4. The method for producing a powder mixture for powder metallurgy according to claim 3, wherein the aliphatic amide is N-oleyl palmitic amide.
 5. The method for producing a powder mixture for powder metallurgy according to claim 3, wherein the aliphatic bisamide is ethylene-bis-oleic amide.
 6. The method for producing a powder mixture for powder metallurgy according to claim 1, wherein the mechanical characteristic-improving powder is composed of at least one element selected from the group consisting of copper, nickel, chromium, molybdenum, graphite and manganese sulfide.
 7. A method for producing a sintered body, comprising the steps of: mixing the iron-base powder and/or the mechanical characteristic-improving powder with an amide-based lubricant solution; removing a solvent in the solution; coating the surface of the iron-base powder and/or the mechanical characteristic-improving powder with an amide-based lubricant; compression-molding the powder mixture for powder metallurgy obtained by the producing method; and sintering.
 8. The method for producing a sintered body according to claim 7, wherein said compression molding is performed at ordinary temperature.
 9. The method for producing a sintered body according to claim 7, wherein 0.01 to 2.0 parts by mass of the amide-based lubricant is added to 100 parts by mass of total amount of the iron-base powder and the mechanical characteristic-improving powder.
 10. The method for producing a sintered body according to claim 7, wherein the amide-based lubricant is composed of an aliphatic amide and/or an aliphatic bisamide.
 11. The method for producing a sintered body according to claim 10, wherein the aliphatic amide is N-oleyl palmitic amide.
 12. The method for producing a sintered body according to claim 10, wherein the aliphatic bisamide is ethylene-bis-oleic amide.
 13. The method for producing a sintered body according to claim 7, wherein the mechanical characteristic-improving powder is composed of at least one element selected from the group consisting of copper, nickel, chromium, molybdenum, graphite and manganese sulfide. 